PROJECT SUMMARY The goal of tobacco regulation is to reduce tobacco-related harm and improve public health. Improving public health in this way requires thorough understanding of the processes underlying purchasing and consumption of the various products available in the complex tobacco marketplace. One product feature eligible for regulation is the nicotine content in electronic cigarettes (e-cigarettes). At present, little research has experimentally examined the influence of nicotine strength on e-cigarette consumption and the likelihood that current cigarette smokers will switch to e-cigarettes (i.e., product substitution). In the absence of such evidence, regulation designed to restrict available e-cigarette nicotine strength may have unanticipated consequences. We will utilize an innovative method, the Experimental Tobacco Marketplace, to provide estimates of the effects of e- cigarette nicotine strength on tobacco consumption and the degree to which e-cigarette products serve as functional substitutes for cigarettes. In so doing, we will provide rigorous tests of a novel quantitative model able to account for e-cigarette substitution effects, including effects of nicotine strength. This model, which assumes that product substitution is directly influenced by the nicotine content of tobacco products relative to their prices (a phenomenon called unit price), provides a framework that may be used to generate novel predictions and guide regulatory efforts. In Experiment 1, we will examine the effects of four e-cigarette nicotine strengths (3, 6, 12, and 24 mg/mL) on product substitution in exclusive cigarette smokers with minimal prior e-cigarette experience. In Experiment 2, we will examine these effects in dual cigarette smokers/e- cigarette users. In both experiments, we will examine possible interactions between nicotine strength and gender on e-cigarette substitution. Together, the findings from this project may be used to inform regulatory action and will improve our understanding of the role of nicotine strength in determining the extent to which e- cigarettes serve as functional substitutes for conventional cigarettes.